1. Field of the Invention
This invention relates to a method and apparatus for controlling the thickness of a liquid coating on a substrate, and more particularly hot dip coating of molten metal on a continuous substrate of ferrous strip. In particular, this invention provides an analytical description of a process wherein an elongated fluid jet is caused to impinge transversely across the entire surface of the upwardly moving coated strip at a distance above the molten coating metal bath, where the coating on the strip is still molten, and where said impinging fluid flow is of such a nature that it forms an effective dam which acts to control the thickness of coating material which is permitted to pass through the impinging jet flow. The present invention provides guide-lines for adjusting the essential control parameters so that optimum performance can be obtained for a given coating system. The analytical description also provides criteria for the design of new coating systems configured to operate at high line speeds, and for the design of effective nozzles. Operation in accordance with the teachings of the invention provides optimum control of coating thickness together with minimization of ripples and build-up of thick edge coatings.
2. Description of the Prior Art
The use of an elongated fluid jet, or so-called "air knife", to control the thickness of coatings applied to paper webs has long been known. Somewhat more recently, the application of such fluid jets to control the thickness of molten metal coatings on metallic substrates has been the subject of considerable investigation. Reference may be made to the following U.S. Pat. Nos. which relate to methods and apparatus for control of molten metal thickness by means of elongated fluid jets:
3,314,163 -- issued Apr. 18, 1967 to J. B. Kohler PA0 3,406,656 -- issued Oct. 22, 1968 to R. W. Patterson PA0 3,459,587 -- issued Aug. 5, 1969 to D. L. Hunter et al PA0 3,480,469 -- issued Nov. 25, 1969 to R. S. Shaffer PA0 3,494,324 -- issued Feb. 10, 1970 to R. L. Bauer et al PA0 3,499,418 -- issued Mar. 10, 1970 to J. T. Mayhew PA0 3,526,204 -- issued Sept. 1, 1970 to P. E. Schnedler et al PA0 3,753,418 -- issued Aug. 21, 1973 to R. Roncan PA0 3,808,033 -- issued Apr. 30, 1974 to J. T. Mayhew
The above-mentioned Mayhew patents disclose an apparatus and method for continuous hot dip metal coating of metallic strip wherein a linearly extended gas outlet means is provided for each surface of the strip, the gas outlet means comprising nozzle means shaped to deliver a concentrated stream of heated gas under pressure across the width of the strip in a direction substantially perpendicular to the opposed planar face thereof, and wherein means is provided for continuously controlling the mass of gas supplied to the nozzle means. Each nozzle means is positioned in substantially direct opposition to the other nozzle means, so that both impinge a stream of gas against opposite planar surfaces of the strip in substantially the same plane.
The nozzle structure disclosed by Mayhew is a two-dimensional converging nozzle incorporating a very narrow, elongated constant-gap channel or throat having a gap or height of 0.005 to 0.015 inch. The channel inlets are square, thereby introducing vena contracta losses. The channel length-to-gap or height ratio is large, thereby causing relatively large frictional pressure losses. The narrow channel gap causes the nozzle to become sonically choked at the flow rates and momentum fluxes necessary for effective jet finishing of molten coating metal and requires the use of high nozzle plenum pressures (20 to 55 psig). Because of the sonic flow at the nozzle exit a static pressure gradient and undesirable lateral expansion or divergence of the free jet would be expected to occur in the region adjacent to the nozzle exit. The high plenum pressures required to overcome the various losses preclude the use of air from conventional multi-stage centrifugal blowers as a jet finishing medium.
In the Mayhew apparatus the distance between the nozzle exit and the pass line of the coated strip varies from about 0.25 to 1.25 inches, and the nozzles may be positioned about 4.5 to 5.5 inches above the coating metal bath level.
The Mayhew patents suggest that line speeds considerably faster than 400 feet per minute should be possible.
In the above-mentioned Hunter et al patent, a molten coating metal weight-control method and apparatus are disclosed, which are alleged to avoid edge build-up or edge bead of coating metal. Broadly, the method comprises impinging gaseous streams (preferably air at ambient temperature) against opposite surfaces of coated strip in such manner that the impingement height of the respective gas streams are overlapping, but offset from each other by an amount of from 1/20 to 3/4 the impingement height.
The Hunter et al. patent teaches the use of a nozzle having a large throat-length-to-gap ratio (similar to Mayhew). Such an elongated channel can be expected to introduce undesirable frictional pressure losses. The nozzle gap is adjustable through the use of variable sized throat section inserts.
The minimum air pressure in the Hunter et al. nozzle is 0.4 psig, exemplary pressures being 15 inches of water (0.55 psi) and 30 inches of water (1.1 psi). It is stated that higher air pressures are required for smaller nozzle orifices (e.g., 0.02 inch) and lower pressures for larger orifices (e.g., 0.25 inch).
Nozzle gap openings between 0.02 inch and about 0.25 inch have proved satisfactory according to Hunter et al. Coating weight is taught to be controlled without impairing the appearance of the coating, while using a given nozzle gap opening, by controlling the flow rate through variations in the nozzle plenum pressure. It is stated that lighter coating weights can be produced at any given line speed by increasing the pressure within the nozzles at a rate that is inversely proportional to the desired change in coating weight. Alternatively, for a given strip speed, nozzle-to-strip distance, and orifice pressure, a decrease in coating weight can be achieved with an increase in the nozzle gap opening.
However, Hunter et al also teaches that for a given orifice height, there is a difference in coating uniformity as the nozzle-to-strip distance is increased. Coatings produced with a nozzle-to-strip distance of 1/2 inch are reported to be uniform in appearance, while those produced with a nozzle-to-strip distance of 1-inch exhibit a faint transverse wave pattern, and those at distances greater than about 11/2 inches exhibit a more pronounced wave pattern. Although nozzle-to-strip distances of about 1/4 to 11/2 inches are stated to be preferred, and nozzle slot heights of 0.06 to 0.15 inch are preferred for galvanizing operations, no guidelines are given between particular values of the nozzle slot height and the resultant ranges in nozzle-to-strip spacings which yield coatings of good appearance. However, it is stated that with all other operating variables being constant, the coating weight of the strip, after passing through the gas streams, can be considered directly proportional to the line speed.
The above-mentioned Roncan patent discloses coating weight control apparatus including the nozzle means comprising a pair of lip members, the lower lip being deformable to vary the longitudinal profile of the nozzle orifices. The nozzle opening in the Roncan structures varies between 0.5 to 1.5 mm (0.02-0.06 inch), the nozzle-to-strip distance between 12 and 18 mm (0.47-0.71 inch), and the air pressure between 600 and 1800 mm of water (0.89-2.67 psi). The height of the nozzles above the coating metal bath varies between 150 and 500 mm (5.9-19.6 inches), and the pair of nozzles on opposite sides of the strip are slightly staggered with respect to each other, with one nozzle impinging perpendicularly on the strip and the other preferably inclined downwardly at an angle of about 80.degree. to the strip, or 10.degree. below horizontal. Ambient air is used as the fluid.
British Pat. No. 1,304,532, dated June 25, 1970, in the name of Armco Steel Corporation (a patent of addition to British Pat. No. 1,221,349), discloses a method of finishing a molten metallic coating on a ferrous metal strip by impinging a laminar flow jet of gaseous fluid on the surface of the coated strip which is maintained flat at the point of impingement the narrow dimension of the fluid jet being contoured progressively from the center thereof outwardly toward each end. Preferably the narrow dimension of the jet is greater at each end than at the center thereof, whereby the coating weight is greater at the center of the strip than at the edges, and heavy edge coatings and oxide "berries" are eliminated.
The above summary of the prior art indicates that numerous suggestions have been offered as to how satisfactory performance might be achieved with a jet finishing system. Numerous variations in nozzle construction and methods of operation have been disclosed. However, the suggested operating guidelines are of a nature which is in general too vague for design and scaling studies, and for optimum operating effectiveness. Furthermore, none of the nozzle designs or suggested operating methods has been completely successful, to the best of applicants' knowledge, in solving the principal problems of reducing ripples in the coating and edge build-up of coating metal, particularly when operating at slow line speeds (e.g., 80 to 170 feet per minute), in achieving precise control of coating thickness across the entire strip width, and a reduction of the noise level associated with fluid jet nozzles, which is greatest when directly opposed.
Moreover, in those prior art systems which utilize steam or other heated gas as the fluid jet medium, it is evident that the cost, maintenance and noise associated with steam or heated gas generation are a distinct disadvantage. The same disadvantage is inherent in the use of compressors for providing gas at elevated pressure, even if unheated.